1. Field of the Invention
The present invention relates, generally, to water-in-resin emulsions wherein water is dispersed in discrete droplets throughout a continuous resinous phase comprised of a copolymerizable mixture of an unsaturated polyester resin, which emulsion is designed to be cured to a porous plastic article. More specifically, the present invention relates to an improvement in the nature of the continuous phase of such an emulsion whereby the promoter system for the polyester resin corresponds generally to promoters for polyester resins in a nonaqueous environment.
2. Description of the Prior Art
Water-in-resin emulsions designed to produce porous plastic parts upon the cure of the resin component have been known for quite some time. Conventionally, an emulsion of water dispersed in discrete droplets throughout a continuous phase comprising an unsaturated polyester resin is cured in a mold having the shape of a desired product whereby the water droplets are trapped in the solid, thermoset polyester composition to yield voids or cells therein. By appropriate manipulation of variables, the dispersed aqueous phase may be trapped in non-communicating cells or allowed to escape from the part through a network of communicating cells. Conventional filler materials, such as hydrated alumina, are sometimes also added to the emulsion mixture prior to cure.
There have been some problems with known water-in-resin emulsions of the character aforesaid, particularly in respect of maintaining good emulsion stability before cure of the resin and, following catalysis, insuring an acceptable gel time. Most manufacturers using these emulsions to fabricate porous plastic articles desire as rapid a cure as possible. Typically, gel times of less than five minutes are necessary, less than three minutes desirable, and on the order of one to two minutes most preferred.
Those skilled in this art have determined that the promoters normally used to cure polyester resins do not give satisfactory results when water is emulsified with the resin to form these porous plastic parts. Various workers, such as the inventors in U.S. Pat. No. 4,077,931, have said that cobalt promoters are especially sensitive to the presence of water and, in these emulsions, the amount of cobalt which must be employed to give satisfactory results is considerably more than in nonaqueous environments, ranging up to about 5 to 10 times that typically employed.
The use of substantial quantities of cobalt promoters, such as cobalt octoate or cobalt naphthenate, gives adequate results in promoting the cure of the polyester component of the emulsion. There is, however, a significant disadvantage in that cobalt is a very expensive commodity. Reducing the amount of cobalt needed to cure effectively a water-in-resin emulsion without loss of emulsion stability during cure is very desirable.